Diffusion testing is a release rate test of an active pharmaceutical ingredient in semisolid form as it permeates through a skin-like membrane into solution. Diffusion replicates the process of skin-applied medicine as it permeates the skin into the body for local or systemic action. It is ideal for the quality control of topical preparations. Semisolid preparations such as creams, ointments, and gels must penetrate the layers of the skin to have benefit. Topical pharmaceutical formulations, designed to permeate the skin, require in vitro release-rate testing that can produce reliable and consistent results. Diffusion testing measures the rate that an active pharmaceutical ingredient is released from a semisolid preparation, providing the quality control analyst with critical performance data. Diffusion testing using diffusion cells has become the industry standard due to the pioneering work of Dr. T. J. Franz who developed the “Franz cell.” This device consists of a small-volume, water-jacketed cell that contains a chamber for drug application, a membrane through which the drug may permeate, and a receptor media chamber from which samples may be extracted and analyzed for drug release. Later developments include non-water-jacketed, dry-heat cells such as Teledyne Hanson's Phoenix™ line of diffusion testers.

A traditional diffusion testing system typically has a group of six cells for simultaneous testing of six specimens. A magnetic cell drive controls the mixing of each cell receptor chamber throughout the test, and a circulating bath provides heated water to the jacketed cells to maintain a constant temperature. In newer systems the receptor media is heated directly, also known as a dry-heat cell. Samples are taken from the receptor chamber, and the same amount of media is then replaced to maintain a constant media-membrane interface.

Sampling of the receptor medium can be performed manually or automatically. Teledyne Hanson's legacy manual diffusion testing systems consist of six cells, a cell drive, a speed control, a media replace beaker, a circulating bath, and a manual sampling syringe. The analyst removes samples using the syringe and replaces the medium after each sample is removed. Hanson's new Phoenix™ line of diffusion cell testers operate without a circulating bath, making the system more portable with a smaller footprint.